[Spoilers Extended] Words of mouth: book canon?

I have a question about GRRM supposedly telling the directors of House of Dragons that Aegon the Conqueror had a dragon dream about the end of the world and that why he have to conqueror Westeros? Should that count as book canon or should we wait til he actually penned it down? Now I wonder how many things will never be officially canon because he still haven’t published it yet? Should we take his words of mouth as canon OR does it need to be written in paper and published?

Of course I don’t expect GRRM to reveal this big secret in his books until ADOS. And probably through a Daenerys chapter